Hello and welcome. The SF250 is a very capable heater,, I'd buy it in a heartbeat. The grates are replaceable and available.. the stove costs in excess of $2000, so you have a good deal there. http://www.harmanstoves.com
If the grates are warped, it is usually caused by not keeping the ash pan emptied, which allows the ashes to build up deep enough that they touch and fill the grates. Without fresh air flowing from the ashpan area through the grates, they can overheat..
Using the shaker handle, move the grates back and forth if you can rock them about 1" each way without them jamming against each other, then they will probably work fine for coal.
New grates should be available from Harman. Call your local Harman dealer for a price and availability..
If the stove has been badly overheated, you may have some warped pieces, and this may be a problem or not. As long as the doors open and close, and everything fits, it should be OK.
The SF 250 has a built in heat exchanger, you need to remove the inspection cover and clean it out.
Hope this helps.. Greg L